This invention relates to a calibratable, electromagnetically compensating balance, especially a precision balance, with a weighing and taring device in the form of a pan support and a parallel guide associated therewith. The present invention relates more particularly to such a balance having an analog-digital converter and a display. The invention is concerned additionally with a method of compensating parameters of the calibratable electromechanical balance which influence sensitivity and accuracy. The term "precision balance" as used herein and according to DIN standards includes all types of analytical balance and microbalances.
In all such balances, a reference or a reference parameter must be provided for the incorporated analog-digital converter for comparison, in order to allow a digital signal to be produced which corresponds to the analog measurement signal. In a known embodiment of a calibratable, electromagnetically compensating balance, an electronic reference is used for this purpose, for example a reference diode, with a circuit associated therewith, which can be turned on and off from the outside. This reference technique, however, is very costly from the standpoint of the circuit technology involved and requires extremely accurate components, such as resistors, amplifiers, magnet systems, and the like, which are very expensive. Despite the expense associated with the circuit technology, however, certain long-term effects, such as drift and temperature influences, cannot be eliminated to the required degree, in order to achieve and reliably ensure the required accuracy. Such long-term effects, however, are especially disadvantageous in the case of calibratable balances, since the latter are sealed and it is not possible to recalibrate these balances to improve accuracy within the set calibration periods, generally two years. It is consequently impossible to do so.
In a further known electromagnetically compensating weighing device, which can be made in the form of a calibratable balance, two toroidal coils are provided, one load coil associated with the load part and one reference coil associated with the reference part, attached to a support loaded with a constant weight. The coils are in the same magnetic system, namely in an air gap between a housing made of magnetic material and a permanent magnet disposed centrally therein. Here the two coils are guided movably and in parallel independently of one another by means of resilient supports; deflections, for example during a weighing process, are determined by a pair of zero-position detectors. The coils are returned to their initial positions, using expensive switching arrangements, by corresponding compensating currents, wherein the mechanical forces correspond to the parameters to be compensated and the compensating current is applied periodically to the load coil, controlled by an associated circuit.
In this known weighing device, a reference mass is constantly applied, so that the balance can operate as a quotient meter, wherein the quotient is constantly determined from the mass to be weighed and the reference mass. A device of this sort is very expensive, however, since practically speaking, two balances must be provided in one instrument. In addition, substantial difficulties arise in conjunction with the parallel guidance, since in the case of a coaxial arrangement of the reference mass and the weighing part for example, parallel arms of different lengths are used, which exhibit pronounced deviations as a function of the actual position as well as the installation site of the balance and as a function of ambient conditions, particularly temperature, in the area of the required high accuracy. Moreover, such double parallel guides require an extremely precise choice of materials as well as a very exact manufacture of the parallel arms. A weighing device of this known type is disclosed in German Auslegeschrift No. 2,223,850.
Hence, the principal aims of the present invention are to provide a calibratable, electromechanical balance of the type described hereinabove, a method for compensation of the parameters of these balances which influence sensitivity and accuracy, as well as a device for carrying out the method, wherein, by means of a small additional expenditure on structural and circuit technology, any of the error parameters which influence the weighing results of the calibratable electromagnetic balance, as for example the dependence on the earth's magnetic field which is determined by the installation site, temperature, humidity changes, and the like, the variations in and aging of the structural elements and the supply devices, as well as changes in the geometric, electrical, and electromechanical parameters can be completely eliminated and ruled out.